While I wrote this back, I got an answer from Canaan support who have been helping but think they are leading me the wrong way with this particular unit. They told me this 1166Pro-68 cannot run with 2 boards as it's not designed to, so deliberately disconnecting it causes this issue (but ignored that it has worked once previously and never again) - they supplied some software and told me to use 'Power Mode 4' so I can run it at 64% but FMS can set it which makes me think it's for 12 and 13 series miners.
I reattached the middle board (H2) and the miner works fine and normally, no issues (log attached bottom of message) - so I don't think there is a comms error on the other two boards:
If that's what they said then you can't run this unit with only two hashboard. I don't know about power mode 4 using the FMS app you can only find two options there under switch mode "normal mode and high performance mode" There is no power mode 4 on the FMS app you can maybe try to send a command to adjust the power to these two hashboard check the command below and send this command using FMS app under diagnose and don't forget to replace 1260 below and test if it would work with only 2 hashboard.
[{"command":"ascset", "parameter":"0,hashpower,1260"}]
If you don't have any other choice then run the 3 hashboards but you would need to replace your wall outlet that support 20a or above.
To be honest, they told me a lot of different things and got the impression they weren't listening to me properly. FMS 2.3.3 is the one on their site, they told me to do the firmware update in AvalonBuddy which they sent through and then when I told them FMS didn't have Power Mode, they sent me a newer FMS 2.3.5 which does have that setting but gets this error when attempting to set:
"ip 192.168.0.240 error result:
ASC 0 set failed: Unknown option: power-level-set"
What does that hashpower command you have provided do? Is that a cgminer command for the 'diagnose' part of FMS?
I have all 3 hashboards connected again now in Normal mode which gets around 60-61TH average, but the cable gets slightly warm, the socket gets warm/hot and the plug pins are too hot to hold onto - not that concerning but over time might get too hot and can't risk it. I should add that I'm in the UK so I am using 240V with 13A fuses in the plug and the sockets are rated for 13-14A max and 20A per dual socket in a modern office with recent electrical renovations so it should be better spec than residential.
I will try your command later (can you explain what it is meant to do please)? If there is a way to limit the frequency of the ASICs or max voltage that would also have the same effect I imagine as long as it sets and takes hold permanently. Is there any way of injecting a command to cgminer when the miner controller board boots?